Additive manufacturing apparatus and methods

ABSTRACT

An additive manufacturing apparatus including a build chamber, build platform lowerable in the chamber so layers of flowable material can successively form across the platform, laser for generating a laser beam, scanning unit for directing the laser beam onto each layer to selectively solidify the material and a processor for controlling the scanning unit. The processor controls the scanning unit directs the laser beam to solidify a selected area of material by advancing the laser beam many times along a scan path. On each pass, the laser beam solidifies spaced apart sections of the scan path, each subsequent pass solidifying sections that are located between sections solidified on a previous pass. The processor controls the scanning unit to direct the laser beam to solidify selected area of material by solidifying sub-millimetre sized sections of non-continuously area and in an order such that consecutively solidified sections are spaced apart.

This is a Continuation of application Ser. No. 15/527,676 filed May 17,2017, which in turn is a National Stage Entry of InternationalApplication No. PCT/GB2015/053484 filed on Nov. 17, 2015, which claimspriority to British Application No. 1420717.9 filed on Nov. 21, 2014.The disclosures of the prior applications are hereby incorporated byreference herein in its entirety.

FIELD OF INVENTION

This invention concerns additive manufacturing apparatus and methods inwhich layers of material are solidified in a layer-by-layer manner toform an object. The invention has particular, but not exclusiveapplication, to selective laser solidification apparatus, such asselective laser melting (SLM) and selective laser sintering (SLS)apparatus.

BACKGROUND

Selective laser melting (SLM) and selective laser sintering (SLS)apparatus produce objects through layer-by-layer solidification of amaterial, such as a metal powder material, using a high energy beam,such as a laser beam. A powder layer is formed across a powder bed in abuild chamber by depositing a heap of powder adjacent to the powder bedand spreading the heap of powder with a wiper across (from one side toanother side of) the powder bed to form the layer. A laser beam is thenscanned across areas of the powder layer that correspond to across-section of the object being constructed. The laser beam melts orsinters the powder to form a solidified layer. After selectivesolidification of a layer, the powder bed is lowered by a thickness ofthe newly solidified layer and a further layer of powder is spread overthe surface and solidified, as required. An example of such a device isdisclosed in U.S. Pat. No. 6,042,774.

Typically, the laser beam is scanned across the powder along a scanpath. An arrangement of the scan paths will be defined by a scanstrategy. U.S. Pat. No. 5,155,324 describes a scan strategy comprisingscanning an outline (border) of a part cross-section followed byscanning an interior (core) of the part cross-section. Scanning a borderof the part may improve the resolution, definition and smoothing ofsurfaces of the part.

It is known to use a continuous mode of laser operation, in which thelaser is maintained on whilst the mirrors move to direct the laser spotalong the scan path, or a pulsed mode of laser operation, in which thelaser is pulsed on and off as the mirrors direct the laser spot todifferent locations along the scan path.

The strategy used for scanning a part can affect the thermal loadsgenerated during the build and accuracy of the resultant solidified lineof material.

Excessive, unrestrained thermal stresses created during the build causeswarping and/or curling of the part being built. As solidified materialcools, the temperature gradient across the cooling solidified materialcan cause warping and/or curling of the part. U.S. Pat. No. 5,155,324and US2008/0241392 A1 describe scanning an area in a plurality ofparallel scan paths (raster scan). The direction of the scan paths arerotated between layers to homogenise tensions generated during thebuild. US2008/0241392 A1 extends this concept to scanning in a series ofparallel stripes, wherein each stripe consists of a plurality ofparallel scan paths running perpendicular to a longitudinal direction ofthe stripe. The direction of the stripes are rotated by 67 degreesbetween layers.

US2005/0142024 discloses a scan strategy for reducing thermal loadscomprising successively irradiating individual areas of a layer, whichare at a distance from one another that is greater than or at leastequal to a mean diameter of the individual areas. Each individual areais irradiated in a series of parallel scan paths.

A melt pool generated by the laser is dependent upon the properties ofthe material and the state (powder or solidified) and temperature ofmaterial surrounding the volume being melted. The scan strategy used canaffect the state and temperature of the neighbouring material. Forexample, scanning of the laser spot along a scan path in continuous modeforms a large melt pool that is dragged along just behind the laserspot, resulting in larger, less detailed solidification lines. For somematerials, such as tool steels and aircraft grade super alloys, it canbe difficult to drag the melt pool across the layer in a continuous modeof operation of the laser. These problems can be mitigated by using thelaser beam in the pulsed mode of operation. In particular, setting thetime between pulses to be long enough to allow a previously formed meltpool to cool before forming an adjacent melt pool can result in moreaccurate solidification lines, which may be particularly beneficial forborder scans. However, slowing the scans to this extent cansignificantly increase the time to scan that area/path and therefore,significantly increase the build time.

SUMMARY OF INVENTION

According to a first aspect of the invention there is provided anadditive manufacturing apparatus comprising a build chamber, a buildplatform lowerable in the build chamber such that layers of flowablematerial can be successively formed across the build platform, a laserfor generating a laser beam, a scanning unit for directing the laserbeam onto each layer to selectively solidify the material and aprocessor for controlling the scanning unit.

The processor may be arranged to control the scanning unit to direct thelaser beam to solidify a selected area of material by advancing thelaser beam a plurality of times along a scan path, wherein on each passlong the scan path, the laser beam solidifies spaced apart sections ofthe scan path, each subsequent pass solidifying sections that arelocated between sections solidified on a previous pass.

The additive manufacturing apparatus may comprise a laser source forgenerating a plurality of laser beams, the scanning unit arranged fordirecting the laser beams onto each layer to selectively solidify thematerial, and the processor is arranged to control the scanning unit todirect the laser beams to solidify a selected area of material byconsecutively advancing multiple ones of the laser beams along a scanpath, wherein on a pass of each one of the laser beams along the scanpath, the laser beam solidifies spaced apart sections of the scan pathand a pass of one of the laser beams along the scan path solidifiessections that are located between sections of the scan path solidifiedby another of the laser beams.

The scan path may be a border scan path around a border of the selectedarea. Carrying out such scanning may increase the build time compared toforming a continuous solidification line along the scan path.Accordingly, for a core of the selected area it may be preferable to usea more efficient scanning strategy. However, at the borders of an area,highly accurate melting may be desired and a scanning method accordingto the invention may achieve increased accuracy along the border scans.However, in certain circumstances it may be desirable to use such ascanning strategy for the core of an area to be solidified. For example,for materials that are difficult to process with scanning strategiesthat solidify the material in large continuous lines (hatches), such astool steels and aircraft grade super alloys, such a scanning strategymay also be used for a core of areas to be solidified.

A first pass of the laser beam along the scan path may be in a firstdirection and a subsequent pass, such as second pass, of the or anotherlaser beam along the scan path may be in a second, opposite direction.For example, for a border scan, the first pass may be in aclockwise/anticlockwise direction around the border and the second passmay be in the other of the anticlockwise/clockwise direction.

The processor may be arranged to control the scanning unit to direct thelaser beam to solidify a selected area of the material by solidifyingsub-millimetre sized sections of the area non-continuously and in anorder such that consecutively solidified sections are spaced apart.

In this way, whilst a section previously irradiated by the laser beam isallowed to solidify, a further section, spaced from the previouslyirradiated section, is irradiated with the laser beam. Accordingly,delays in solidification of the selected area are reduced compared towaiting for the previous section to solidify before irradiating theadjacent section whilst inaccuracies and thermal stresses formed bycontinuous scanning of large sections are avoided.

With such small sections, a more isotropic solidified section may beformed compared to longer sections. It will be understood that“sub-millimetre sized section” means that the all dimensions of thesection are less than 1 mm.

Each section may be formed from irradiating a single point with the(static) laser beam or moving the laser beam across the layer, forexample in the formation of a line. A size of the section may be set bythe time it takes for the material to solidify. In one embodiment, thesection may be sized such that irradiation of the section with the laserbeam results in a melt pool extending across the entire section.

The processor may be arranged to control the scanning unit to direct thelaser beam to solidify a selected area of the material by irradiatingsections of the area with the laser beam such that each irradiatedsection is allowed to solidify before an adjacent section is irradiatedwith the or another laser beam.

Each section of a selected area of one layer may be arranged to (only)partially overlap with sections of a corresponding selected area of aprevious layer. Each section may be a substantially round spot, thespots of each layer arranged in a regular pattern, wherein the patternof one layer is offset relative to a corresponding pattern of theprevious layer. The spots may be arranged in a triangular pattern. Thespots of the pattern may be solidified in an order such that adjacentspots are not sequentially solidified.

Irradiation of the spots of the pattern with the or a plurality of laserbeams may progress across the pattern in a direction different to adirection that irradiation of spots progressed (in a like manner) acrossa pattern in the corresponding selected area of the previous layer. Fora triangular pattern, the direction in which irradiation of the spotsprogresses may be changed by 60 or 120 degrees between each layer.

According to a second aspect of the invention there is provided a methodof scanning layers of material in a layer-by-layer additivemanufacturing process, wherein successive layers of flowable materialare formed across a build platform and a laser beam scanned acrossselected areas of each layer to solidify the material in the selectedareas.

The method may comprise directing the laser beam to solidify a selectedarea of the material by solidifying sub-millimetre sized sections of thearea non-continuously and in an order such that consecutively solidifiedsections are spaced apart.

The method may comprise directing the laser beam to solidify a selectedarea of material by advancing the laser beam a plurality of times alonga scan path, wherein on each pass long the scan path, the laser beamsolidifies spaced apart sections of the scan path, each subsequent passsolidifying sections that are located between sections solidified on aprevious pass.

The method may comprise directing a plurality of laser beams to solidifya selected area of material by consecutively advancing multiple ones ofthe laser beams along a scan path, wherein on a pass of each one of thelaser beams along the scan path, the laser beam solidifies spaced apartsections of the scan path and a pass of one of the laser beams along thescan path solidifies sections that are located between sections of thescan path solidified by another of the laser beams.

The method may comprise directing the laser beam to solidify a selectedarea of the material by irradiating sections of the area with the laserbeam such that each irradiated section is allowed to solidify before anadjacent section is irradiated with the laser beam, wherein each sectionof a selected area of one layer is arranged to (only) partially overlapwith sections of a corresponding selected area of a previous layer.

According to a third aspect of the invention there is provided a datacarrier having instructions stored thereon, which, when executed by aprocessing unit of an additive manufacturing apparatus, cause theprocessing unit to control the additive manufacturing apparatus to carryout the method of the second aspect of the invention.

The data carrier of the above aspects of the invention may be a suitablemedium for providing a machine with instructions such as non-transientdata carrier, for example a floppy disk, a CD ROM, a DVD ROM/RAM(including -R/-RW and +R/+RW), an HD DVD, a Blu Ray™ disc, a memory(such as a Memory Stick™, an SD card, a compact flash card, or thelike), a disc drive (such as a hard disc drive), a tape, anymagneto/optical storage, or a transient data carrier, such as a signalon a wire or fibre optic or a wireless signal, for example a signalssent over a wired or wireless network (such as an Internet download, anFTP transfer, or the like).

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a selective laser solidification apparatusaccording to an embodiment of the invention;

FIG. 2 is a schematic of the selective laser solidification apparatusfrom another side;

FIGS. 3a and 3b are schematic diagrams illustrating scans along a scanpath;

FIG. 4 is a schematic diagram illustrating a fill scan of an area inaccordance with an embodiment of the invention; and

FIG. 5 is a schematic diagram illustrating the fill scan of FIG. 4 formultiple layers.

DESCRIPTION OF EMBODIMENTS

Referring to FIGS. 1 and 2, a laser solidification apparatus accordingto an embodiment of the invention comprises a main chamber 101 havingtherein partitions 115, 116 that define a build chamber 117 and asurface onto which powder can be deposited. A build platform 102 isprovided for supporting an object 103 built by selective laser meltingpowder 104. The platform 102 can be lowered within the build chamber 117as successive layers of the object 103 are formed. A build volumeavailable is defined by the extent to which the build platform 102 canbe lowered into the build chamber 117.

Layers of powder 104 are formed as the object 103 is built by dispensingapparatus 108 and an elongate wiper 109. For example, the dispensingapparatus 108 may be apparatus as described in WO2010/007396.

A laser module 105 generates a laser for melting the powder 104, thelaser directed as required by optical scanner 106 under the control of acomputer 130. The laser enters the chamber 101 via a window 107.

The optical scanner 106 comprises steering optics, in this embodiment,two movable mirrors 106 a, 106 b for directing the laser beam to thedesired location on the powder bed 104 and focussing optics, in thisembodiment a pair of movable lenses 106 c, 106 d, for adjusting a focallength of the laser beam. Motors (not shown) drive movement of themirrors 106 a and lenses 106 b, 106 c, the motors controlled byprocessor 131.

Computer 130 comprises the processor unit 131, memory 132, display 133,user input device 134, such as a keyboard, touch screen, etc, a dataconnection to modules of the laser melting unit, such as optical module106 and laser module 105 and an external data connection 135. Stored onmemory 132 is a computer program that instructs the processing unit tocarry out the method as now described.

Processor receives via external connection 135 geometric data describingscan paths to take in solidifying areas of powder in each powder layer.To build a part, the processor controls the scanner 106 to direct thelaser beam in accordance with the scan paths defined in the geometricdata.

Referring to FIGS. 3a and 3b , in this embodiment, to perform a scanalong a scan path, such as a border scan path 200, extending around anarea of material to be solidified, the laser 105 and scanner 106 aresynchronised to expose a series of discrete points 201 along the scanpath 200 to the laser beam. For each scan path 200, a point distance, d,point exposure time and spot size is defined. A direction, D, in whichthe points 201 are scanned is also defined. In FIG. 3a , the border scanpath 200 is scanned twice in the direction D, with spaced apart points201 a, shown in FIG. 3a with a horizontal line fill, exposed on a firstpass of the laser beam along the scan path 200 and spaced apart point201 b, shown in FIG. 3a with a dotted fill, in between points 201 a,exposed on a second pass along the scan path 200. In FIG. 3a the points201 (melt pools formed by the laser beam) are shown as not overlappingfor clarity but, in practical applications would at least slightlyoverlap such that a line of solidified material is formed along the scanpath 200.

By successively solidifying every other point 201 along the scan path,the material melted at each point 201 is allowed to solidify beforematerial at an adjacent point is solidified, during which time the laserbeam melts material at other points 201. Allowing a melt formed at eachpoint 201 to solidify separately may allow more accurate solidificationlines to be formed. In particular, a melt front is not dragged aroundthe scan path 200 by the laser beam, which can result in inaccuraciesand epitaxial or columnar grain growth.

In the embodiment shown in FIG. 3a , when each point 201 is beingirradiated by the laser beam, the laser beam spot is held substantiallystationary at the point 201, forming a substantially spherical meltpool. However, as illustrated in FIG. 3b , some of the accuracyadvantages may still be achieved by forming spaced apart elongate meltpools less than 1 mm in length though small movements of the laser beamspot across the powder bed before the laser beam is turned off andjumped to the next spaced apart section 210, 211 of a scan path 200 tobe exposed. Forming elongate sections 210, 211 rather than discretepoints 201 may be desirable in order to balance accuracy againstefficiency.

It is believed that, for typical laser parameters used in selectivelaser melting, the laser beam can irradiate sections of less than 1 mmto form a melt pool extending across the entire length of the section.In this way, the solidified section 210. 211 will have few directionalproperties. Beyond 1 mm, the start of a section will solidify before anend of the section has been melted. Metal material typically solidifieswithin 0.1 to 1.66 μs. A speed of the laser beam is dictated by theenergy that the laser beam can couple into the material within a unitperiod of time whilst at the same time avoiding excessive vaporisationof the material. For a 500 Watt laser focussed to an 80 micron spot, thespeed of the laser beam can be of the order of 2 to 500 m/s.

In FIG. 3b , in a first pass along the scan path 200, the laser beamirradiates spaced apart sections 210 and on a second pass of the scanpath, the laser beam irradiates spaced apart section 211 that liebetween sections 210.

In both FIGS. 3a and 3b , both the first pass and the second pass are inthe same direction. However, in an alternative embodiment, the firstpass and the second pass are in opposite directions. Furthermore, in yetanother embodiment, the points or sections are spaced such that three ofmore passes have to be made along the scan path to form a continuoussolidification line along the scan path.

FIG. 4 shows a further scan strategy according to an embodiment of theinvention for solidifying a core of an area 303. Points 301 areirradiated by the laser beam to solidify area 303. The points 301 arearranged in a 2-dimensional triangular pattern and the laser irradiatesthe points in an order, indicated by numbers 1 through 28 such thatsuccessively irradiated points 301 are spaced apart and a point (orpossibly points) between the successively irradiated points is (are)irradiated after the successively irradiated points have had time tosolidify or irradiated and solidified before the successively irradiatedpoints are irradiated.

In the order shown in FIG. 4, the points 301 are scanned along linearscan paths (each column of points 301) in one of two directionsindicated by the arrow, D. Spaced apart points 301 in a first scan path(far left column) are scanned in a first direction (down the page) andthen spaced apart points 301 in a second scan path (column second fromthe right) are scanned in a second, opposite direction (up the page).The laser beam then returns to the first scan path to scan the spacedapart points 301 located between the points 301 previously irradiated onthe first pass along the first scan path. This continues for all scanpaths (columns of points in direction, D) until the entire area 303 hasbeen solidified.

It will be understood that like FIG. 3b , rather than points 301, thecore may be filled with separately irradiated elongate sections.Furthermore, rather than each pass along a scan path being in the samedirection, each pass along the scan path may be in opposite directions.

FIG. 5 shows the fill pattern for three successive layers 402 a to 402c. The location of the points 401 for each layer 402 a to 402 c areoffset relative to the adjacent layers such that the centres of thepoints 401 of adjacent layers do not coincide. The linear scan paths ofpoints 401 for each layer 402 a to 402 c are scanned in directions asindicated by arrows D₁, D₂, D₃, the directions being rotated betweeneach layer 402 a to 402 c. In this embodiment, the triangle pattern ofthe points 401 allows the directions to be rotated by 60 degrees betweeneach layer.

Rather than progressing along linear scan paths in one of two opposeddirections as described above, the scanning sequence for the triangularpattern of points 301, 401, shown in FIGS. 4 and 5 may progress withmovement of the laser beam in orthogonal directions. In this way,successively irradiated points 301, 401 of the pattern are located apartin direction, D, and in a direction orthogonal to D.

Furthermore, the offset patterns shown in FIG. 5 may still providebenefits even with a scanning sequence in which adjacent points aresuccessively irradiated.

It will be understood that alterations and modifications may be made tothe above described embodiments without departing from the scope of theinvention as defined herein. For example, the additive manufacturingapparatus may comprise a plurality of laser beams and scanning modulesfor independently steering each laser beam. In the embodiment shown inFIGS. 3a and 3b , each pass along the border scan path may be by thesame or a different laser beam. In particular, a second laser beam maystart scanning along the scan path before the first laser beam hascompleted scanning of the laser path, the two scans sufficiently spacedsuch that sections irradiated by the first laser beam have solidified bythe time the second laser beam begins solidifying adjacent sectionsalong the scan path. In the second embodiment shown in FIGS. 4 and 5,various scanning strategies could be used with multiple laser beams. Thelaser beams could be scanned along the same paths or alternatively, morecomplex scanning strategies could be used, wherein each laser beam isadvanced along a different scan path (which may or may not overlap inpart).

1. An additive manufacturing apparatus comprising a build chamber, abuild platform lowerable in the build chamber such that layers offlowable material can be successively formed across the build platform,a laser for generating a laser beam, a scanning unit for directing thelaser beam onto each layer to selectively solidify the material and aprocessor for controlling the scanning unit, the processor arranged tocontrol the scanning unit to direct the laser beam to solidify aselected area of material by advancing the laser beam a plurality oftimes along a scan path, wherein on each pass along the scan path, thelaser beam solidifies spaced apart sections of the scan path, eachsubsequent pass solidifying sections that are located between sectionssolidified on a previous pass.
 2. A laser melting additive manufacturingapparatus comprising: a build chamber; a build platform lowerable in thebuild chamber such that layers of powder can be successively formedacross the build platform; a laser for generating a laser beam; ascanning unit for directing the laser beam onto each layer toselectively solidify powder; and a processor for controlling thescanning unit, the processor arranged to control the scanning unit todirect the laser beam to consolidate a selected area of a layer of thepowder by melting sections of the area non-continuously and in an ordersuch that consecutively melted sections are spaced apart, the meltedsections disposed in a two-dimensional arrangement, wherein at least oneof the melted sections comprise at least three adjacent sections,wherein the melted sections are sub-millimetre sized such that the meltpool extends across the entire melted section and each of the meltedsections is allowed to solidify before an adjacent section is melted byirradiating the layer with the or another laser beam.
 3. A laser meltingadditive manufacturing apparatus according to claim 2, wherein eachsection is formed from irradiating a single point with the laser beam.4. A laser melting additive manufacturing apparatus according to claim2, wherein each section is formed by moving the laser beam across thelayer.
 5. A laser melting additive manufacturing apparatus according toclaim 2, wherein the two-dimensional arrangement of melted sections is aregular pattern.
 6. A laser melting additive manufacturing apparatusaccording to claim 5, wherein the regular pattern is a triangularpattern.
 7. A laser melting additive manufacturing apparatus accordingto claim 1, wherein the two-dimensional arrangement of melted sectionssolidify to form a core of an object.
 8. A laser melting additivemanufacturing apparatus according to claim 1, wherein thetwo-dimensional arrangement of melted sections comprises a fill scanpattern for forming a core of an object.
 9. A laser melting additivemanufacturing apparatus according to claim 1, wherein the sections ofthe layer are arranged such that a centre of each section is offsetrelative to centres of sections of the adjacent layers.
 10. A method ofscanning layers of powder in a layer-by-layer laser melting additivemanufacturing process, wherein successive layers of powder are formedacross a build platform and a laser beam scanned across selected areasof each layer to consolidate powder in the selected areas, the methodcomprising: directing the laser beam to melt a selected area of thelayer by melting sections of the area non-continuously and in an ordersuch that consecutively melted sections are spaced apart, the meltedsections disposed in a two-dimensional arrangement, wherein at least oneof the melted sections overlaps with at least three adjacent sections;and allowing each of the melted sections to solidify before directingthe or another laser beam to melt an adjacent section of the layer,wherein the melted sections are sub-millimetre sized such that the meltpool extends across the entire melted section.
 11. A method according toclaim 10, wherein each section is formed from irradiating a single pointwith the laser beam.
 12. A method according to claim 10, wherein eachsection is formed by moving the laser beam across the layer.
 13. Amethod according to claim 10, wherein the two-dimensional arrangement ofmelted sections is a regular pattern.
 14. A method according to claim13, wherein the regular pattern is a triangular pattern.
 15. A methodaccording to claim 10, wherein the two-dimensional arrangement of meltedsections solidify to form a core of an object.
 16. A method according toclaim 10, wherein the two-dimensional arrangement of melted sectionscomprises a fill scan pattern for forming a core of an object.
 17. Amethod according to claim 10, wherein the sections of the layer arearranged such that a centre of each section is offset relative tocentres of sections of the adjacent layers.
 18. A data carrier havinginstructions stored thereon, which, when executed by a processing unitof an additive manufacturing apparatus, cause the processing unit tocontrol the additive manufacturing apparatus to carry out the method ofclaim 10.